Designing a Circular Future in the Solar Industry

Solar energy is helping reduce the United States’ dependence on fossil fuel, but solar manufacturers have neglected design changes that would benefit recycling technologies for modules once they hit their 25-year lifespan.  

Paul Leu, B.P. Faculty Fellow and professor of industrial engineering at the University of Pittsburgh Swanson School of Engineering, is part of a team of researchers who are concerned with the lack of attention to circularity in the solar industry. They say that current recycling efforts aren’t focusing on design changes that would increase solar module circularity.

Circularity focuses on reuse and waste elimination at the end of a product’s life cycle. For solar module manufacturers, circular design changes increase the efficiency of recycling component materials for future use.

“Without the necessary design changes, current recycling technologies are only reclaiming 10-15 percent of silicon module material by weight for reuse in new modules,” Leu said.

The research team is proposing five new design changes to silicon solar modules to create a more circular solar industry:

1. Eliminating environmental hazards with lead-free modules
2. Utilizing silver-free modules to control recycling costs
3. A new encapsulant for easy separation & exposure of silicon cells from modules
4. An effective recycling technology or new encapsulant for dual-glass modules
5. Traceability of module information to facilitate effective reuse and recycling

Eliminating environmental hazards with lead-free modules

Silicon modules contain approximately 10-15 grams of toxic lead, yet few recyclers are interested in removing it - less because of lead’s environmental impact, but rather more because of the cost.

In silicon module production, the solder used to electrically interconnect copper wires is manufactured using 60 percent tin and 40 percent lead. Lead produces an environmental hazard if not removed from recycling sledge before landfilling, but module owners can utilize more environmentally friendly lead-free modules, made of approximately 96 percent tin, 3 percent silver, and 1 percent copper. All metals in this module can be recovered through recycling.

There are, however, still challenges. Adopting the lead-free solder costs an additional $0.35 per module and requires a higher melting point than the leaded solder. More stress and the chance of silicon wafer breakage are the primary concerns of a higher melting point.

Utilizing silver-free modules to control recycling costs

Not only is silver the most expensive material in silicon cells, but it’s also the source of 40 percent of total revenue in silicon module recycling. As the solar industry continues to skyrocket, so does the metal’s price, making it egregiously expensive for manufacturers.  

“Over the last decade, there’s been a four-fold reduction in silver content on a per-watt-peak basis, and another two-fold reduction is proposed by 2030,” said Meng Tao, professor at Arizona State University’s School of Electrical, Computer and Energy Engineering. “At this point, the only way for recyclers to recover silver is to charge the extra cost to module owners, which leads to a higher recycling fee.”

A possible solution is to replace silver with copper. Electroplated copper is a proven replacement, but the industry hasn’t adopted this technique because of the extra patterning step’s significant process cost. A copper paste would eliminate the need for patterning since it works the same way as the industry’s current silver paste – though copper still has its challenges.

Using a copper paste can potentially kill cell efficiency once it diffuses into silicon. Despite the significant advantages like abundance and cost-efficiency, manufacturers would need to find a way around copper oxidation and diffusion during the high-temperature firing over a 25-year lifespan.

Creating a new silicon cell encapsulant for easier separation and exposure from modules

A new encapsulant would allow easy separation and exposure of silicon cells from modules. Delamination, the act of splitting or separating a laminate into layers, is currently the biggest barrier to silicon module recycling.

Module production involves encapsulating silicon solar cells with a covalently crosslinked polymer known as ethylene vinyl acetate (EVA), which binds cells to glass and a back sheet.

Because EVA is difficult to break down, it prevents easy separation of silicon cells from the glass and back sheet. No tools commercially available are capable, either. The only practical removal technique is thermal decomposition in a furnace above 500 degrees Celsius.

Polymer encapsulants not covalently crosslinked like EVA are possible substitutes for easier delamination. Thermoplastic polyolefins (TPOs) are crosslinked through van der Waals bonding and undergo reversible thermophysical softening and hardening through heating and cooling. However, TPOs are prone to moisture permeability and can lack strong adhesion to silicon cells.

Self-healing and stimuli-responsive polymers for solar modules are at an early stage but indicate a design strategy that could improve encapsulant delamination and recyclability of silicon modules.

Dual-glass modules

Dual-glass modules are expected to increase in solar installations. They have a higher power output because each side has a glass sheet, so both sides of the module are producing power. Glass sheets are more durable, mechanically stronger, and sustain less degradation than a polymer back sheet.

The main issue? There’s no method to separate silicon cells from either of the glass sheets.

“Even existing tools like NPC’s hot knife, which is a blade heated to 300 degrees Celsius to separate glass from other materials, don’t work on a rigid glass back sheet,” Tao said. “That means all of the silver, lead, silicon, tin, and copper metals can’t be recovered and downcycling is needed for glass with silicon cells still attached.”

Module traceability

By the time a module hits the end of its life in the field, its label often becomes unreadable making its product specifications difficult to find. Recyclers are forced to go through a difficult and costly investigative process to confirm design, construction, and related product materials to ensure effective recycling.

“Industry standard unique identifiers can be applied by manufacturers to solve this problem,” Leu stated. “Unique IDs, like smart tags and quick-response codes, enable module traceability and are linked back to a standardized data model that includes ratings, specifications, and constituent materials. It would also account for low-content materials like silver, lead, and tin in silicon modules.”

Any recycler or supply chain stakeholder would have the ability to obtain module information within and after its 25-year lifespan. Unique ID standards and supporting technologies are being developed with funding from the United States Department of Energy.

“The research community needs to work with industry partners to emphasize design changes that improve module circularity,” Leu said. “These proposed design changes can help reclaim up to 95 percent of the materials by weight for reuse in new modules.”

Other researchers on this project include: 

• Meng Tao, professor of engineering at Arizona State University
• Thad Druffel, theme leader, solar manufacturing R&D, Conn Center for Renewable Energy Research at the University of Louisville
• Alicia Farag, co-founder and president at LocusView Solutions
• Kim McLoughlin, principal engineer at Braskem’s Innovation and Technology Center 

The paper, “Design changes for improved circularity of silicon solar modules” (DOI: 0.1016/j.oneear.2024.01.020), was published in the journal One Earth in February 2024.